Soot oxidation catalyst and method of making

ABSTRACT

A catalyst composition for facilitating the oxidation of soot from diesel engine exhaust is provided. The catalyst composition includes a catalytic metal selected from Pt, Pd, Pt—Pd, Ag, or combinations thereof, an active metal oxide component containing Cu and La, and a support selected from alumina, silica, zirconia, or combinations thereof. The platinum group metal loading of the composition is less than about 20 g/ft 3 . The catalyst composition may be provided on a diesel particulate filter by impregnating the filter with an alumina, silica or zirconia sol solution modified with glycerol and/or saccharose, impregnating the filter with a stabilizing solution, and impregnating the filter with a solution containing the active metal oxide precursor(s) and the catalytic metal precursor(s). The resulting catalyst coated diesel particulate filter provides effective soot oxidation, exhibits good thermal stability, has a high BET surface area, and exhibits minimal backpressure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/385,005 filed Mar. 20, 2006 now U.S. Pat. No. 7,771,669, entitledSOOT OXIDATION CATALYST AND METHOD OF MAKING. The entire contents ofsaid application is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a catalyst composition for thecombustion of particulates from diesel engine exhaust, and moreparticularly, to a catalyst composition which facilitates the oxidationof soot during the regeneration of diesel particulate filters and whichcontains little or no platinum group metals.

In recent years, environmental regulations in the United States andEurope restricting diesel particulate emissions have necessitatedimprovements in the removal of particulates from diesel engineemissions. Such particulates generally consist of carbonaceousparticulates in the form of soot. Currently, the most commonly usedmethod for removing soot from engine exhaust is the use of a dieselparticulate filter (“DPF”) which collects the soot, followed byoxidation of the accumulated particulates at elevated temperatures whichregenerates the filter.

The first types of diesel particulate filters were installed in urbanbuses and passenger cars as early as 1986. However, a problem with theuse of such diesel particulate filters is that regeneration of thefilter by soot oxidation is difficult due to the low temperatures ofdiesel exhaust gases (e.g., less than about 200° C.), which areunfavorable for soot oxidation. Accordingly, it is necessary to useperiodic high-temperature regeneration of the filter to oxidize the sootat elevated temperatures.

More recently, a particulate filter system has been used in Europeandiesel passenger cars which relies on fuel-borne catalytic assistance inthe regeneration of the DPF, i.e., the inclusion of a catalyst,typically metal, in the fuel as an additive which functions to lower thetemperature at which carbon combusts. However, such a system is complexand requires additional components such as a tank for fuel additives, anadditive dosing system, and infrastructure to refill the additive fueltank. In addition, the use of fuel-borne catalysts can lead to theformation of ash which accumulates on the filter, causing gradual lossof filter soot capacity and a decrease in time between regenerationevents. Therefore, it is necessary to change the filter after aboutevery 80 K kilometers.

Also known in the art are “continuously regenerating traps” (CRT®),which comprise a platinum-based diesel oxidation catalyst (“DOC”)positioned upstream of a diesel particulate filter. Such a trap uses NO₂generated on the Pt-containing DOC for soot oxidation. However, whenused in diesel passenger cars, the amount of NO_(x) emitted isinsufficient to provide complete soot oxidation due to lower NO_(x)/sootratio. Accordingly, it is still necessary to use periodichigh-temperature regeneration of the filter using oxygen from air tooxidize the soot at elevated temperatures.

Another known method for removing soot is to deposit a catalyst on thewalls of the DPF, also referred to as a “catalyzed DPF.” A catalyzedsoot filter typically comprises one or more platinum group metalcatalysts and is less complex than fuel-borne catalysts. See, forexample, WO 00/29726, EP 0160482, EP 0164881 and U.S. Pat. No.5,100,632. See also WO 01/12320, which teaches the use of platinum orpalladium-containing oxidation catalysts within a diesel particulatefilter to improve soot oxidation on the filter.

Catalyzed DPFs containing platinum group metals are also described in DE10214343A1, U.S. Pat. No. 4,900,517, and EP1055805. See also U.S. Pat.No. 5,746,989, EP 0758713, JP 2003278536, and EP1398069, which teach adiesel particulate NO_(x) reduction system utilizing platinum groupmetals. See also US 2002-127252, EP 0658369B1, U.S. Pat. No. 5,330,945,U.S. Pat. No. 4,759,918, U.S. Pat. No. 5,610,117, U.S. Pat. No.5,911,961, U.S. Pat. No. 6,143,691 and JP 11253757, which teachparticulate filters or traps containing platinum group metals.

However, in the catalyzed DPF system, the contact between the catalystand soot is relatively loose, resulting in low catalyst activity forsoot oxidation. Thus, in the catalyzed DPF system, it is more difficultto oxidize soot, and the oxidation typically requires highertemperatures. Further, regeneration of the filter requires that thecatalyst be able to withstand temperatures of up to 1000 to 1200° C.This is a problem for noble metal catalysts as intensive sintering ofsuch catalysts occurs at temperatures above 750° C.

Another problem with catalyzed diesel particulate filters currently inuse is that they typically employ platinum group metal (PGM)compositions, particularly platinum-based formulations which areprovided in the form of a catalytic coating, or washcoat. Such a coatingis very expensive to manufacture due to the high cost of platinum groupmetals. Accordingly, it would be desirable to be able to eliminate allor part of the platinum group metals used in diesel particulate filters.

In addition, catalyst coatings containing platinum group metals arehighly active and may result in undesirable reactions such as oxidationof SO₂ to SO₃, and the formation of sulfated ash and sulfatedparticulate. In order to minimize these side effects, many catalystsuppliers have tried to decrease the concentration of platinum groupmetals in catalyst coatings. However, this leads to lower activities insoot oxidation, thus compromising efficiency.

In commonly-assigned (Ford) application EP 1356864, a catalyticcomposition is taught for soot oxidation which is free of platinum groupmetals. However, such a composition is adapted for use in soot oxidationin the presence of NO_(x) as a soot oxidant, not to high-temperaturesoot oxidation in the presence of oxygen.

Accordingly, there is still a need in the art for a catalyst which canbe used in a diesel particulate filter, which contains little or noplatinum group metals, and which can effectively oxidize soot by oxygenduring periodic high temperature regenerations.

SUMMARY OF THE INVENTION

The present invention meets those needs by providing a catalystcomposition for use in a diesel particulate filter which uses little orno platinum group metals (PGM), and which effectively oxidizes soot.

According to one aspect of the present invention, a catalyst compositionis provided for facilitating soot oxidation which comprises a catalyticmetal selected from Pt, Pd, Pt—Pd, Ag, or combinations thereof and anactive metal oxide component containing Cu and La; wherein when thecatalytic metal comprises a platinum group metal, the platinum groupmetal (PGM) loading in the composition is less than about 20 g/ft³.Preferably, the PGM loading is about 15 g/ft³.

Preferably, the catalyst composition further includes a support selectedfrom alumina, silica, zirconia, and combinations thereof. Preferably,the support material has been stabilized with lanthanum, zirconium,aluminum, or combinations thereof. By “stabilized,” it is meant that thesupport material is prevented from sintering at high temperatures, i.e.,temperatures greater than about 1000 to 1200° C. which may beencountered during regeneration of a diesel particulate filter.

The active metal oxide component preferably comprises CuO—La₂CuO₄, whichprovides good soot oxidation properties to the catalyst composition.

The catalyst composition of the present invention is preferably providedon a diesel particulate filter or other porous substrate for providingoxidation of soot accumulated on the filter. In a preferred method ofproviding the catalyst on the filter, a diesel particulate filter isfirst impregnated with a colloidal solution selected from alumina sol,silica sol, zirconia sol, or combinations thereof, which, when dried,forms a support material for the catalyst. Preferably, the colloidalsolution further includes an organic compound selected from saccharose,glycerol, and combinations thereof. Such organic compounds function toimprove the surface area of the coating. After coating the filter withthe colloidal solution, the impregnated filter is then dried andcalcined.

The filter is then impregnated with a stabilizing solution to preventsintering of the support. Where the colloidal solution comprises analumina sol, the stabilizing solution comprises zirconyl acetate,lanthanum nitrate, or a combination thereof. Where the colloidalsolution comprises silica sol, the stabilizing solution compriseszirconyl acetate, aluminum nitrate, or a combination thereof. Where thecolloidal solution comprises zirconia sol, the stabilizing solutioncomprises lanthanum nitrate.

After impregnation with the stabilizing solution, the coated filter isagain dried and calcined. Next, the filter is impregnated with asolution containing a metal oxide precursor selected from coppernitrate, lanthanum nitrate, and mixtures thereof, and a catalytic metalprecursor selected from silver nitrate, palladium nitrate, dihydrogenhexachloroplatinate, and combinations thereof. The impregnated filter isthen dried and calcined.

Where the diesel particulate filter has been coated with a compositionwhich includes a colloidal solution comprising alumina sol or silicasol, the coated filter preferably has a BET surface area of at least30-40 m²/g. Where the filter has been coated with a compositioncontaining a zirconia sol, the coated filter preferably has a BETsurface area of at least 9-12 m²/g.

The present invention also provides a diesel exhaust gas treatmentsystem comprising a diesel particulate filter for receiving dieselexhaust gas from a diesel engine and a catalyst composition impregnatedin the filter, where the catalyst composition comprises a catalyticmetal selected from Pt, Pd, Pt—Pd, Ag, or combinations thereof and anactive metal oxide component containing Cu and La; where the platinumgroup metal loading in the catalyst composition is less than about 20g/ft³.

When incorporated in such a diesel exhaust treatment system, thecatalyst composition of the present invention is capable of oxidizingsoot at a temperature of between about 550° C. and 600° C.

Accordingly, it is a feature of the present invention to provide acatalyst composition having little or no platinum group metal loadingwhich provides effective oxidation of soot. Other features andadvantages will be apparent from the following description and theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating soot oxidation activity for metaloxide-containing catalysts made in accordance with the presentinvention;

FIGS. 2A and 2B are graphs illustrating the regeneration efficiency ofwashcoated filters of the present invention compared with commerciallycoated filters;

FIG. 3 is a graph illustrating the pressure drop balance temperature ofthe washcoated filters of the present invention compared withcommercially coated filters;

FIG. 4 is a graph illustrating soot oxidation activity duringregeneration of washcoated filters of the present invention comparedwith commercially coated filters;

FIG. 5 is a graph illustrating the maximum temperatures reached duringregeneration at the highest soot loading;

FIGS. 6A and 6B are graphs illustrating CO selectivity during sootoxidation for washcoated filters of the present invention, commerciallycoated filters, and uncoated filters; and

FIG. 7 is a graph illustrating the backpressure of washcoated filters ofthe present invention and uncoated filters.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

We have found that a synergistic effect results from the combination ofthe catalytic metals Ag, Pt and/or Pd with the metal oxide componentCuO—La₂CuO₄ for use as a catalyst in soot oxidation. This combinationdecreases the temperature necessary for soot oxidation while decreasingthe loading of costly Pt and Pd. While not wishing to be bound by anyparticular theory, we believe that the synergetic effect resulting fromthe combination may be due to the presence of the noble metal whichfacilitates partial copper reduction to induce the copper-containingsystem to be more active in soot oxidation, while the copper containingcompound functions as an oxygen supplier for the noble metals.

The catalyst composition of the present invention provides comparable orbetter soot oxidation properties than commercial formulations havingmuch higher noble metal loadings. In addition, when coated onto a dieselparticulate filter, the catalyst composition provides good thermalstability, minimal backpressure, and high surface area. By “thermalstability” it is meant that the catalyst maintains its activity oroxidation performance even after exposure to high temperatures. By“backpressure,” it is meant the resistance to flow which is caused whenthe filter pores become blocked or are decreased in size, e.g., from theaccumulation of soot or the coating of the catalyst onto the filter. By“minimal backpressure” it is meant that, when coated onto the filter,the catalyst coating of the present invention increases the backpressureof the filter only slightly.

Preferred catalytic metals for use in the catalyst composition includePt, Pd, Pt—Pd, Ag, or combinations thereof. The preferred metal oxidecomponent has a Cu/La ratio of 1:1 and preferably comprises CuO—La₂CuO₄

The catalyst composition of the present invention is preferablysupported on a refractory oxide to increase its surface area, andconsequently, its activity in soot oxidation. The refractory oxidesupport also functions to increase the thermal stability of the catalystto high temperatures encountered in diesel particulate filterapplications. Preferred refractory oxides for use in the presentinvention include alumina, silica, and zirconia.

Such refractory oxides are preferably deposited on a diesel particulatefilter by impregnating the filter with a colloidal solution selectedfrom alumina sol, silica sol, zirconia sol, or combinations thereof. Wehave found that impregnating diesel particulate filters with such a solsolution avoids blocking the pores of the filter (monolith), resultingin minimal backpressure. While coating with a slurry has been commonlyused for coating of filters (monoliths) with catalysts such as 3-waycatalysts, a slurry is not feasible for coating on a diesel particulatefilter as the average pore diameter of a silicon carbide (SiC) substratematerial is about 9 microns. Coating such a substrate material with aslurry would block the pores or significantly decrease their diameterbecause the size of particles in such a slurry is comparable or higherthan the size of the pores in the filter.

While sols have been used for monolith coatings in automotive catalysis,typically only about 5 wt. % sol has been used for the purpose allowinglarger slurry particles to adhere to the surface. However, we have foundthat the sol solution itself can provide good high surface area coatingof the catalyst on porous filter materials such as diesel particulatefilters.

The average particle size of the sols is preferably 0.004 to 0.01microns for silica sol, 0.05 microns for alumina sol, and 0.005-0.001microns for zirconia sol. Preferred sols for use in the presentinvention are commercially available from Alfa Aesar. The amount of solused will vary depending on the porosity of the substrate material.Typically, about 520-550 ml of sol is used for a standard filter size of5.66 inches×6 inches having a porosity of 65% (made from cordierite)while about 350 ml of sol is used for silicon carbide filters having aporosity of 42%.

We have found that in a typical porous filter material (dieselparticulate filter made from cordierite or SiC), after absorption of 300to 550 ml of solution (5.66″×6″ filter), the BET surface area is about24-30 m²/g after one coating with 20% alumina sol or 30% silica sol. Itis preferred that the particle size of the sol solutions be at least 2orders lower than the filter pore size to avoid blocking of the pores.

We have additionally found that the surface area of the catalyst coatingis significantly improved by adding organic compounds such as saccharoseand glycerol to the sol solution (see Table 2). Such organic compoundsalso prevent the increase in size of primary particles in the alumina,zirconia or silica sols. Saccharose functions as a templating agent,while glycerol functions to prevent the sticking of primary particles.Both compounds additionally decrease the crystallization of aluminum andthe collapse of the monolith structure during the drying step. It shouldbe noted that pure sugar may be used instead of saccharose with the sameresults. Preferably, 5-10 g of saccharose and 10-30 ml of glycerol areadded to 1 L of sol solution. While higher concentrations will furtherincrease the surface area, we have found that such higher concentrationsalso increase the solution viscosity, which is undesirable for coatingpurposes.

After application of the sol on the filter by impregnation, the filteris preferably dried at a temperature of about 77 to 100° C. and calcinedat a temperature of about 285° C. for about 1 hour to form the supportfor the catalyst.

In order to maintain the surface area of the sol coating and to preventsintering of the support at high temperatures (e.g., regenerationtemperatures up to 1200° C.), it is preferable to stabilize the alumina,silica, or zirconia support material after it is coated onto themonolith and dried/calcined. Where the support comprises alumina, thealumina is stabilized by impregnation with zirconium and/or lanthanumprecursors, preferably zirconyl acetate and lanthanum nitrate.Preferably, 10-15% Zr (mol % to Al) and 3-5 La (mol % to Al) is used.Where the support comprises silica, a solution of zirconyl acetate ispreferred for stabilization. Preferably, 10-20% Zr (mol % to Si) isused. In addition, aluminum (aluminum nitrate) may be added to furtherincrease the thermal stability of the support, up to about 5 mol %.Where the support comprises zirconia, a lanthanum solution is used.Preferably, 3-5% La (mol % to Zr) is used. After application of thestabilizing solution, the coated filter is again dried at about 77 to100° C. and calcined at about 800° C. for about 2 hours.

After stabilization, the filter is then impregnated with a washcoatsolution containing the active metal oxide precursor selected fromcopper nitrate, lanthanum nitrate, and mixtures thereof, and thecatalytic metal precursor selected from silver nitrate, palladiumnitrate, and dihydrogen hexachloroplatinate, and combinations thereof.

The catalyst washcoat solution preferably further includes a smallamount of citric acid, preferably about 35-40 g per filter, which aidsin providing a homogeneous deposition of the catalyst on the filter.

The washcoat solution may also include one or more nitrates selectedfrom yttrium nitrate, magnesium nitrate, iron nitrate, cerium nitrate,and cobalt nitrate. The nitrates are preferably added in small amountsof about 1-4 g to further stabilize the support and/or the catalystagainst sintering.

After impregnation of the catalyst washcoat, the coated filter ispreferably calcined at about 750° C. for about 16 hours.

Thus, the catalyst composition of the present invention may be providedon a diesel particulate filter in a three-step process in which thefilter is 1) impregnated with a colloidal solution, 2) impregnated witha stabilizing solution, and 3) impregnated with a catalyst washcoat.While the catalyst composition is described herein as being coated ontoa diesel particulate filter, it should be appreciated that thecomposition may also be provided on other porous substrates such asmullite, aluminum titanate, alumina, etc. The catalyst composition ispreferably coated onto a diesel particulate filter comprised of porouscordierite or silicon carbide (SiC).

Where the diesel particulate filter is comprised of SiC, thecatalyst-coated filter preferably has a BET surface area of at least30-40 m²/g where the support material comprises alumina or silica. Wherethe support material comprises zirconia, the BET surface area is about9-12 m²/g.

The diesel particulate filter including the catalyst composition thereinmay be used in a diesel exhaust gas treatment system to provideeffective soot oxidation. The catalyst composition preferably providesfrom about 60 to 100% soot oxidation at a targeted temperature ofregeneration between about 550 and 600° C., and more preferably,provides about 80 to 100% soot oxidation. Typically, duringregeneration, additional fuel is injected into the engine, and a dieseloxidation catalyst positioned upstream from the diesel particulatefilter combusts the additional fuel, resulting in the release of heat.This heat increases the temperature of the diesel particulate filter upto about 600° C., which is sufficient to oxidize any accumulated soot onthe filter.

In order that the invention may be more readily understood, reference ismade to the following examples which are intended to illustrate theinvention, but not limit the scope thereof.

Example 1 Preparation of Catalyst Composition on Powder Samples

A catalyst composition was prepared in accordance with the presentinvention. A silicon carbide (SiC) powder was prepared by grindingcommercial SiC square units to form a filter substrate material having aporosity of 60%. The SiC powder obtained was then sieved to obtain afraction of 0.1 to 0.125 mm. This SiC powder was then coated with asolution containing alumina sol and saccharose (pure crystal sugar) andglycerol. The samples were dried 100° C. and calcined at 625° C. for 1hour.

The alumina was then stabilized by impregnation with solutions ofzirconyl acetate and lanthanum nitrate (3-5% La and 10-15% Zr; molar %of La and Zr to Al) followed by drying and calcining at 800° C. for 2hours.

Next, the samples were impregnated with a water solution containing thecatalyst precursor salts, i.e., copper nitrate, lanthanum nitrate, andcitric acid and the precious metal precursors, e.g., silver nitrate,palladium nitrate, and dihydrogen hexachloroplatinate (H₂PtCl₆,chloroplatinic acid). The samples were then calcined at 750° C. for 16hours.

Example 2 Thermogravimetric and Flow Reactor Testing

Catalyst compositions prepared in accordance with the present inventionwere characterized using a thermogravimetric analysis-mass spectrometry(TGA-MS) method using a Hiden IGA-003 analyzer equipped with a massspectrometer. In the procedure used, 240 mg of SiC powder with adeposited catalyst (20 wt % catalyst loading on powdered SiC (Ibiden)high porosity substrate) and 60 mg of real diesel soot were used. Thecatalyst/soot weight ratio was 4/1. Prior to testing, the catalyst andsoot were mixed in a mortar with a spatula to provide loose contactbetween the catalyst and soot to simulate typical soot-catalyst contenton a catalyzed filter. The samples were first heated in a reaction gas(6% O₂—He; flow rate 100 cc/min) from 40° C. to 200° C. with a heatingrate of 20° C./min. and maintained at 200° C. for 20 minutes to removeall soluble organic fraction of soot (SOF), and then heated from 200° C.to 600° C. at 20° C./min. After reaching 600° C., the sample was keptunder isothermal conditions for 30 minutes and then cooled down. The gasleaving the reactor was analyzed for CO/N₂, H₂O, O₂ and CO₂ usingquadruple MS Hiden HPR-20. The indicated sample temperature was measuredby a thermocouple placed at approximately 1 cm distance from thecrucible.

Initially, more than 100 different oxide and mixed oxide catalystswithout noble metals were evaluated on their activity in soot oxidationat 500-650° C. by the TGA-MS method. Among all catalysts tested, onlyCu, Ag, and Cr-containing compositions were found to be active in sootoxidation below 600° C. as can be seen in FIG. 1. The activity ofcopper-containing catalysts was higher than that of silver and chromiumcompounds. The most active composition at low temperatures was La₂CuO₄.This compound is known to have a K₂NiF₄ structure (See Y. Wu et al., J.Molec. Catalysis A: 155 (2000) pp. 89-100 and S. D. Peter et al.,Catalysis Letters 54 (1998) pp. 79-84).

Copper oxide (CuO) was also active, especially at 550° C., butdemonstrated lower activity in comparison with lanthanum-copper mixedoxide La₂CuO₄ at lower temperatures, while its activity was higher thanLa₂CuO₄ at 550° C. (see FIG. 1). In attempting to combine the bestproperties provided by CuO and La₂CuO₄, we further tested the solidsolutions of CuO—La₂CuO₄. As La₂CuO₄ is the only compound in the copperoxide-lanthanum oxide binary system, the formation of solid solutions ofCuO—La₂CuO₄ is typical when the concentration of copper oxide is higherthan in La₂CuO₄, e.g., the Cu/La molar ratio is higher than 1:2. (see“Handbook of status diagrams for systems for hardly melting metals,”S-Petersburg, Nauka (Science) 1997). Such solid solutions demonstratedthe best soot oxidation properties exceeding those of the individualcomponents, CuO and La₂CuO₄.

The CuO—La₂CuO₄ compound (Cu/La ratio 1/1) was found to be the mostactive compound as shown in FIG. 1. It was also noted that this solidsolution oxidized soot without any oxygen in the gas phase, i.e., up to30% of soot was oxidized using only nitrogen flow at 550-600° C., so thecatalyst was able to oxidize soot using only its matrix oxygen. Thisproperty is very valuable for high soot loading of the filter, whenoxygen access through the pores of the filter (when loaded with soot) islimited and soot oxidation is slow due to the limited oxygen supply.

The copper oxide-lanthanum oxide solid solution and different noblemetal compositions including combinations of noble metals andCuO—La₂CuO₄ were also tested in a flow reactor to determine theiractivity in soot oxidation. The quartz flow reactor was equipped with anelectrical heater and temperature-programmed controller. The reactiongas mixture was 10% O₂ in nitrogen. The soot and powder catalyst weremechanically mixed using a spatula to imitate loose contact typical forsoot oxidation on a filter. The ratio of catalyst to soot was 4/1 (40 mgcatalyst to 10 mg of soot). The reactor effluent gases were analyzedusing an electrochemical analyzer “Testo-33” (Eco-Intech),thermochemical sensor, and gas chromatography. The gas flow rate usedwas 30 ml/min.

Catalyst PM-2 was used as a reference sample modeling commercialplatinum-containing compositions. Samples PM-3, PM-9 and PM-12 were alsotested as reference Pt-containing compositions. The results are shownbelow in Table 1.

TABLE 1 Soot conversion on different catalysts in reaction mixture of10% O₂ in nitrogen, 10 mg soot and 40 mg of catalyst, feed flow rate 30ml/min., temperature ramp 10° C./min. All samples were prepared onstabilized alumina-washcoated SiC powder as described above. 10% 50% 90%soot soot conversion soot conversion conversion Sample ID (° C.) (° C.)(° C.) PM-1 Cu—Co spinel 490 558 610 PM-2 Pt/CeO₂ 458 542 593 PM-3Pt—CoLaO₃ 470 546 592 perovskite PM-4 Cu—Ce—Fe 465 543 600 PM-5Pt—CuO—La₂CuO₄ 458 528 578 PM-6 Ag—CuO—La₂CuO₄ 438 519 578 PM-7 CuO 457547 602 PM-8 Pd—CuO—La₂CuO₄ 438 511 568 PM-9 Pd—CeO₂ 472 563 611 PM-10CuO—La₂CuO₄ 457 532 592 PM-11 Ag/CeO₂ 473 560 610 PM-12 Pt—Co—Mn 465 546596 PM-13 La₂CuO₄ 460 533 598

As can be seen, the CuO—La₂CuO₄ composition (PM10) showed activitycomparable with the activity of platinum group metal (PGM) catalysts,including the reference PM-2, PM-3, PM-9 and PM-12 samples. Also asshown, the combination of Pd, Pt and Ag with CuO—La₂CuO₄ (PM-5, 6 and 8)as used in the present invention demonstrated the lowest temperatures ofsoot oxidation initiation, as evaluated from the temperature of 10% sootconversion, average rate of soot oxidation (which can be evaluated from50% soot conversion), and temperature of near complete soot oxidation,which can be evaluated from 90% soot conversion. It is believed that thereason for the synergy between the platinum group metal or Ag andCu-containing oxide component is that the precious metal componentfacilitates partial copper oxide reduction which is favorable for sootoxidation, while the copper-based oxide component functions as an oxygensupplier for the metal component.

Example 3 Coating Method for Filters

A number of porous substrates were coated with an alumina sol, azirconia sol, or a silica sol containing various amounts of glyceroland/or saccharose. The BET surface areas of the coated substrates werethen measured by N₂ adsorption at 77K using Micromeritics 2010 ASAPequipment. The results are shown below in Table 2.

TABLE 2 Number of Substrate/porosity Coating coatings (%) BET, m²/g 20%alumina sol 1 Cordierite - 65% 24 20% alumina sol 2 Cordierite - 65% 3120% alumina sol + glycerol 1 Cordierite - 65% 32 20% alumina sol +saccharose 1 Cordierite - 65% 29 20% alumina sol + glycerol + 1Cordierite - 65% 37 saccharose 20% alumina sol + glycerol + 2Cordierite - 65% 56 saccharose 30% silica sol 1 Cordierite - 65% 31 30%silica sol + glycerol + 1 Cordierite - 65% 40 saccharose 20% zirconiasol 1 Cordierite - 65% 7 20% zirconia sol + glycerol + 1 Cordierite -65% 12 saccharose 20% alumina sol 1 SiC - 60% 19 20% alumina sol +glycerol + 1 SiC - 60% 30 saccharose 20% alumina sol 1 Composite - 85%54 20% alumina sol + glycerol + 1 Composite - 85% 94 saccharose 30%silica sol 1 Composite - 85% 75 30% silica sol + glycerol + 1Composite - 85% 89 saccharose

As can be seen, the BET surface area after coating with the sol solutionmodified with glycerol and/or saccharose exceeded the surface area for acommercial coating which was in the range of 12 to 25 m²/g. As can beseen, the higher the porosity of the filter material, the higher thesurface area obtained. Thus, an advantage of the coating method is thatit allows for 1.5 to 2 times higher surface area, e.g. a surface area upto 90-95 m²/g for the most porous filter material.

Example 4 Stabilization of Supports

The alumina and silica samples of Example 3 were then stabilized aftercoating and calcination as follows. The alumina samples were impregnatedwith a solution containing zirconium and lanthanum precursors, i.e.,zirconyl acetate and lanthanum nitrate. The silica samples wereimpregnated with zirconia using a solution of zirconyl acetate.Generally, 10-15% Zr (mol % to Al) and 3-5 La (mol % to Al) were usedfor stabilization of alumina and 10-20% Zr (mol % to Si) was used forstabilization of silica. The samples containing stabilized andnon-stabilized alumina and silica on cordierite were then calcined atdifferent temperatures and the specific surface area of alumina andsilica was calculated from the data and is shown in Table 3 below.

TABLE 3 BET surface area, m²/g after calcinations for 2 hours 1050° C.,12 hours for Al₂O₃ 1000° C., Support 800° C. 900° C. 2 hours for SiO₂SiO₂ 422 206 3 10% Zr—SiO₂ 402 330 96 5% Al—20% Zr/SiO₂ 335 282 159Al₂O₃ 211 176 18 3% La—Al₂O₃ 202 192 109 3% La—15% Zr/Al₂O₃ 184 178 154

As can be seen, the thermal stability of alumina and silica was greatlyimproved at temperatures above 900° C. by stabilization using La and/orZr.

Example 5 Catalyst Coating of Commercial Diesel Particulate Filters

Washcoats prepared in accordance with the present invention were coatedonto commercially available diesel particulate filters made from siliconcarbide and having 200 cpsi cell density, a porosity of 42% and 5.66″×6″in size. The method of preparing the washcoats was the same as inExample 1 except that a more diluted sol (typically 10% alumina sol or15% silica sol) was used due to the lower porosity (42%) of the filtersused. Typically, a 350 ml solution was applied to the DPFs, which isclose to the water absorption capacity of the filters. The washcoatswere coated onto a number of filters as described in further detailbelow.

Filter #55

Initial weight of the (uncoated) filter was 1921 g. First, the filterwas impregnated with 10% alumina sol, modified with 10 ml of glyceroland 2.7 g of saccharose, dried at 100° C. overnight and calcined at 285°C. for 1 hour. Then, the filter was impregnated with a solutioncontaining lanthanum nitrate (5% mol to alumina, i.e., a molar ratio ofLa/Al of 5 molecules La to 100 molecules of Al) and zirconyl acetate(15% mol alumina) followed by drying at 100° C. and calcining at 800° C.for 2 hours. The filter was then impregnated with a solution containingdihydrogen hexachloroplatinate (IV), copper nitrate and lanthanumnitrate, and small amounts of yttrium and magnesium nitrates and citricacid (38 g). Pt loading was 1.32 g (15 g/ft³), Cu loading was 4.4 g,La₂O₃ loading was 6.4 g, MgO was 1.6 g and Y₂O₃ was 1.9 g. The filterwas then calcined at 750° C. for 16 hours prior to further testing. Thefinal weight of the filter was 1995 g.

Filter #56

Initial weight of the filter was 1919 g. The filter was prepared usingthe same impregnation, drying, and calcining conditions as describedabove with regard to Filter #55 except that 8% alumina sol was used, andLa and Zr loading were 3 and 10% mol of alumina weight. A mixedPt—Pd—CuO—La₂CuO₄ composition was used as the catalyst, using a solutioncontaining palladium nitrate, dihydrogen hexachloroplatinate, coppernitrate, lanthanum nitrate, and a small amount of iron (III) nitrate. Ptloading was 0.88 g (10 g/ft³), Cu loading was 3.6 g, La₂O₃ loading was4.7 g, and Fe₂O₃ was 1.5 g. The final weight of the filter was 1979 g.

Filter #57

Initial weight of the filter was 1972 g. The filter was prepared asdescribed above with regard to Filter #55 except that a 17.5% aluminasol was used. The catalyst comprised a solution containing palladiumnitrate, copper nitrate, small amounts of iron and cerium nitrate. Pdloading was 1.32 g (15 g/ft³), CuO loading was 9.6 g, La₂O₃ loading was6.5 g, Fe₂O₃ was 1.5 g, and CeO₂ was 2.1 g. The final weight of thefilter was 2073 g.

Filter #58

Initial weight of the filter was 1918 g. First, the filter wasimpregnated with a 35% solution of ammonium zirconium carbonatecontaining urea, and then dried at 100° C. and calcined at 257° C. for 1hour. The zirconia coating was then stabilized by impregnation withlanthanum nitrate and calcination at 800° C. for 2 hours. Finally, acatalyst solution containing dihydrogen hexachloroplatinate, coppernitrate and lanthanum nitrate with a small amount of iron nitrate andcitric acid was applied to the filter. The filter was then dried at 100°C. and calcined at 750° C. for 16 hours. Pd loading was 1.32 g (15g/ft³), CuO loading was 5.4 g, La₂O₃ loading was 4.3 g, and Fe₂O₃ was 1g. The final weight of the filter was 1949 g.

Filter #59

Initial weight of the filter was 1944 g. The filter was impregnated witha 10% alumina sol, and stabilized with Zr and La as described above withregard to filter #56. A catalyst solution containing silver nitrate,copper nitrate, lanthanum nitrate and small amounts of cerium nitrate,iron nitrate and manganese nitrate, and citric acid was applied to thefilter. The filter was dried at 77° C. and calcined at 750° C. for 16hours. Ag loading was 20 g, CuO loading was 11.1 g, La₂O₃ loading was7.7 g, CeO₂ was 2.1 g, Fe₂O₃ was 1.5 g, and Mn₂O₃ was 1.25 g. The finalweight of the filter was 2044 g.

Filter #60

Initial weight of the filter was 1974 g. The filter was impregnated witha 15% silica sol containing 10 ml glycerol and 1.7 g saccharose, driedat 107° C. and calcined at 287° C. The silica coating was stabilizedwith 20% mol Zr from zirconyl acetate and 5 mol % of alumina fromaluminum nitrate, then dried and calcined at 775° C. for 2 hours. Thecatalyst solution included palladium nitrate, dihydrogenhexachloroplatinate, copper nitrate, lanthanum nitrate and small amountsof cobalt nitrate, cerium nitrate, and iron nitrate, followed by dryingat 77° C. and calcination at 750° C. for 16 hours. Pd loading was 0.66 g(7.5 g/ft³), Pt loading was also 0.66 (7.5 g/ft³), CuO loading was 8.0g, La₂O₃ loading was 7.0 g, CO₃O₄ was 2.9 g, CeO₂ was 2.1 g, and Fe₂O₃was 1.5 g. The final weight of the filter was 2074 g.

Example 6 Engine Testing Procedure

The coated sample filters were then tested on an engine dynamometer with2.0 L common rail (CR) diesel engine equipped with a commercial dieseloxidation catalyst (DOC) in a close-coupled position. Distance betweenthe DOC rear and the DPF front was approximately 10 cm. Fuel with asulfur level of 500 ppm was used for comparative tests. The prototypesof commercial Pt-coated DPFs with Pt loading of 50-150 g per cubic feetwere tested as reference DPFs, and an uncoated DPF made from SiC havingthe same porosity, size and cell density was also tested as a referenceDPF.

The test procedure was as follows: 1) high temperature cleaning offilter at 600-610° C.; 2) pressure drop characterization and filterweighing under warm conditions (about 200° C.); 3) soot loading at 260°C.+/−15° C.; 4) pressure drop characterization and filter weighing underwarm conditions; 5) regeneration with targeted pre-DPF temperature; and6) pressure drop characterization and filter weighing under warmconditions.

Under the test conditions used, the degree of pressure drop recovery andsoot mass oxidized during regeneration calculated from weight differencewere adequate indicators of the regeneration performance.

The sequence of tests for each filter was as follows: 1) steady-stateregeneration for 15 minutes at pre-DPF temperature of 550° C. and masssoot loading of 6 g/L to evaluate initial soot activity; 2) fourregenerations with soot loading increasing from 4 to 10 g/L (4, 6, 8, 10g/L) using a “drop-to-idle” scenario (i.e., the engine was switched offafter the beginning of regeneration, providing low flow through thefilter and high oxygen concentration, causing the highest peaktemperature during regeneration) to evaluate filter regeneration undersevere conditions, i.e., conditions favorable for development of highpeak temperatures inside DPF; 3) steady-state regeneration for 15minutes at pre-DPF temperature of 550° C. and mass soot loading of 6 g/Lto evaluate activity of aged catalyst after all regenerations.

The activity of the filters was compared with the activity of threecommercial prototype filters having Pt loading of 50 g/ft³ (50 MPF 53),100 g/ft³ (100 MPF 32) and 150 g/ft³ (150 MPF 85), respectively. A firstcomparison was made using the effectiveness of regenerations withincreasing soot loading. The effectiveness was evaluated by measuringthe filter weight before and after regenerations, and mass of soot stillremaining inside the filter was calculated. As can be seen in FIG. 2,the washcoated filters of the present invention exhibited regenerationcharacteristics comparable with commercial coatings, despite the lowerprecious metal loading. Filter #56 (Pt—Pd—CuO—La₂CuO₄) showed nearcomplete soot regeneration under any soot loading. Filter #57 (Pd only)showed a similar high regeneration effectiveness. Other filters wereless effective at low soot loading, but their behavior was stillcomparable or better than the commercial prototypes as can be seen inFIG. 2.

Another important characteristic value of soot oxidation is the balancepressure drop temperature, i.e., the temperature at which no furtherincrease of pressure drop can be found, e.g., dP/dT=0. At this point,the decrease of dP by soot combustion compensates the increase of dP dueto the increase of temperature and space velocity. A lower balancepressure drop temperature indicates higher activity in soot oxidation atlower temperatures or more effective soot oxidation ignition. Thepressure drop balance temperature of the filters during regeneration isillustrated in FIG. 3. The commercial prototypes demonstratedessentially the same behavior and their balance temperatures are shownby a dashed curve. As the prototypes' balance points are essentially thesame as that of an uncoated filter, it can be seen that there is noadvantage of using such prototypes for the initiation of soot oxidationin comparison with an uncoated filter. In contrast, both Pt—CuO—La₂CuO₄filters (#55 and #58) and Pt—Pd—CuO—La₂CuO₄ (#56) showed much lowerbalance point temperatures, indicating more effective initiation of sootcombustion. The effect was less pronounced on filters #57(Pd—CuO—La₂CuO₄), #59 (Ag—CuO—La₂CuO₄) and #60 (silica sol coating).

The reactivity of the soot filters in soot oxidation was also evaluatedusing steady-state regenerations at 550° C. and soot loading of near 6g/L. The first regeneration was used to evaluate the initial oxidationactivity, and the last regeneration was used to evaluate soot oxidationactivity after all drop-to-idle regenerations with soot loadings varyingfrom 4 to 10 g/L, and to evaluate the catalyst survival after suchsevere regenerations. The reactivity in soot oxidation was calculatedfrom experimental data using Arrhenius equations. The activity in sootoxidation with oxygen was estimated for each filter using the equation:Δm _(soot) =M _(soot) ^(2/3)×(K _(O2)[O₂ ]+k _(NOx)[NO_(x)])−Δm _(SR)where M=soot mass, K_(O2)=constant of soot oxidation with oxygen,[O₂]=oxygen concentration, kNO_(x)=constant of soot oxidation withnitrogen oxides, [NO_(x)]═NO_(x) concentration, and Δm_(SR)=increase ofsoot mass due to the soot coming from the engine. The contribution ofsoot mass decrease from soot oxidation with NO_(x) was generally verysmall. The constant of activity in soot oxidation with oxygen wasevaluated for each filter from equation:K _(O2) =K ⁰ _(O2) ×T×exp(E _(act) /T),where T=temperature, E_(act)=activation energy of soot oxidation withoxygen, and E_(act)=18040K.

The values for soot oxidation activity are shown in FIG. 4. As can beseen from the data, the initial activity of the filters of the presentinvention (#55-60), preliminarily calcined at 750° C. for 16 hours, ishigher or comparable than commercial prototypes having higher PGM (Pt)loading. The K⁰ _(O2) values are also generally higher than forcommercial prototypes, even for PGM-free (Ag—CuO—La₂CuO₄) filter #59.The initial activity of filter #56 (Pd—Pt—CuO—La₂CuO₄) showed the bestresults among the filters studied.

The maximum temperatures inside of DPFs tested are shown in FIG. 5. Asshown, the temperatures reached 1100 to 1200° C. during regenerationevents at highest soot loading. The results indicate that generally, theactivity in soot oxidation decreased after 5 regenerations for filtersof the present invention as well as the commercial prototypes, althoughonly by about 10%. However, this decrease in activity was not found forfilters #55 (Pt—CuO—La₂CuO₄/stabilized alumina) and #57(Pd—CuO—La₂CuO₄/stabilized alumina) of the present invention. The use ofsilica as a support appears to be less effective to prevent catalystdeactivation as can be seen for filter #60.

CO selectivity during soot oxidation on different filters is illustratedin FIG. 6. The uncoated filter released the highest amount of CO duringregeneration, which is typical for the gas-phase oxidation of sootwithout catalytic assistance. CO is not a desirable product duringfilter regeneration; however, CO oxidation to CO₂ leads to a largerelease of heat and an increase in peak temperatures during regenerationthat decreases the safety of regenerations. As can be seen in FIG. 5,the maximum temperatures are always higher in catalyzed filters. Withregard to CO oxidation to CO₂, it would be expected that the filters ofthe present invention would not perform as well as commercial prototypeswhich have from 3 to 10 times higher PGM loading. However, all of thefilters of the present invention have shown significant CO reductionduring regeneration, comparable to that of a commercial filter havingthe lowest Pt loading among the commercial prototypes (50 g/ft³).Filters #55, 56 and 57 were especially effective with a CO selectivityof less than about 10%. Filter #59 was less effective in CO oxidationdue to the absence of active PGM but still provided some CO oxidation.Filter #58 (zirconia support) demonstrated higher CO oxidation,presumably because of the lower surface area of the support.

Referring now to FIG. 7, a comparison of backpressure of the filters isillustrated. The filters coated in accordance with the present inventionhave a low increase of filter backpressure. The backpressure of thecoated filters was within 19-40 mBar, while the backpressure was 24 mBarfor an uncoated filter. The backpressure is also dependent on thesupport and catalyst loading; e.g., the backpressure was minimal forlow-loaded filter #58 (40 g support+catalyst loading) and was near 40mBar for the most heavily loaded filters #57 and 60 (87-101 gsupport+catalyst loading). Generally, the backpressure of filters of thepresent invention was lower than for commercial prototypes, havingbackpressures in the range of 44-110 mBar, typically 55-80 mBar. Thisillustrates that the method of coating the filter in accordance with thepresent invention provides lower backpressure than filters provided witha commercial catalyst coating.

While certain representative embodiments and details have been shown forpurposes of illustrating the invention, it will be apparent to thoseskilled in the art that various changes in the methods and apparatusdisclosed herein may be made without departing from the scope of theinvention, which is defined in the appended claims.

What is claimed is:
 1. An oxidation catalyst consisting essentially ofCu-containing alumina impregnated with platinum.
 2. An exhaust gastreatment system comprising: a) an exhaust gas system through whichexhaust gas flows; and b) an oxidation catalyst consisting essentiallyof Cu-containing alumina impregnated with platinum.